1. Field of the Invention
The present invention relates to an image forming apparatus comprising a sheet feeder, installed in an image reading apparatus, such as a copying machine and a scanner, or in an image forming apparatus, such as a printer and a facsimile machine, for taking out sheets such as documents stored in a stacked state in sheet storing means one by one and supplying and conveying the sheets to a sheet conveyance path.
2. Description of Related Art
In recent years, a device that automatically conveys documents in the form of sheets to a reading apparatus and reads the documents one by one is installed in an image forming apparatus or the like, and efficient image reading or image forming operations are performed. Further, with the development of digital technologies, the document reading speed, the conversion speed to electronic data and the image forming speed from electronic data become faster, and the number of sheets of documents to be set at a time in such a reading apparatus is significantly increased to around 100 to 200 sheets so as to process a greater amount of documents at high speeds. In addition, with the improvement of document conveying means of the document reading apparatus, types of documents that can be conveyed is increased.
In an image reading apparatus that supplies and conveys a large amount of sets sheets such as documents and reads the images of the documents, a tray on which the sheets are mounted is kept on standby in a state capable of loading a maximum number of sheets, elevation of the tray is started when a reading start signal is inputted by a key operation or the like to start reading documents, and an operation to supply documents to a predetermined conveyance path is performed. Therefore, it takes a long time to reach a state capable of supplying the documents to the conveyance path by elevating the tray, and there is the problem of an increase in the total reading time required to complete the reading of documents.
In order to solve this problem, control is performed to automatically elevate the tray after a predetermined time has elapsed from the detection of a document mounted on the tray and complete the elevation of the tray before a reading start signal is inputted, thereby shortening the total reading time. Such an image reading apparatus is disclosed, for example, in Japanese Patent Application Laid-Open Nos. 11-237771 (1999), 10-250853 (1998) and 7-17640 (1995).
However, with the sheet feeder that automatically elevates the sheet storing unit until the topmost layer of sheets comes into contact with the sheet conveying means by simply detecting that the sheets were mounted in the sheet storing unit, the user may be confused since the elevation of the sheet storing unit may be started while a document regulating plate for regulating the document mount position is being manipulated.
Moreover, the sheet storing means may be elevated before the sheet mount position is properly regulated and the topmost layer of sheets may come into contact with the sheet conveying means, and thus troubles occur easily. In other words, if the elevation of the tray is started when setting documents on the tray, the position of the document regulating plate for aligning the document conveying position is displaced, and the set condition of documents becomes imperfect. If the documents are conveyed in the imperfectly set condition and the images are read, there arise problems of displacement or tilting of the position of the image.